Sir Nicholas Henderson: Diplomat who secured American help for Britain during the Falklands War

Sir "Nicko" Henderson was in the great tradition of splendidly flamboyant British diplomats: colourful but effective, larger than life but with an underlying shrewdness, idiosyncratic without being actually eccentric.

He was one of British diplomacy's natural charmers, marked out by effortless social graces, an extraordinary network of connections and an instinctive ability to establish good relations with almost everyone. More than one observer has thought of him as a fugitive from previous centuries, and indeed he would have excelled at the Congress of Vienna, where his close-quarter skills could have been deployed to maximum advantage.

Instead he spent much of his long career involved in working on Britain's place in post-war Europe, attempting to build better relationships with Germany and France and urging London to accept that it was a power in steady decline. At the age of 60 he signed off with a famous despatch which lamented that Britain had become "poor and unproud". Yet suddenly he found himself propelled, by Margaret Thatcher, into the diplomatic front line as ambassador to Washington.

Once there he found himself embroiled in the Falklands War, acting as a key figure making the arguments for the British cause both to the Reagan administration and, in scores of television interviews, to American public opinion. Some in the US found Mrs Thatcher a shade too belligerent for their taste. The suave Henderson, with his well-dressed but somehow forever rumpled elegance, and understated, temperate manner, provided a valuable counterpoint.

In Washington some thought the US should turn down Britain's requests for assistance in the dispute, arguing that to do so could only harm America's relations with various countries in the region. Henderson worked to ensure that the interventionist line of the Defence Secretary, Caspar Weinberger, prevailed in the Oval Office, with the result that the Reagan administration provided generally surreptitious but hugely helpful and possibly decisive aid.

The Thatcher-Henderson double act was all the more remarkable because of their differences in other areas. He was, for example, strongly committed to the pro-European camp (though intriguingly he was later to say she was initially quite europhile.)

His father, the economist and academic Sir Hubert Henderson, was a Liberal, as were many of Henderson's early friends and associates: others lent to the Fabian left. The family were on the fringes of the Bloomsbury set and Sir Hubert went on to become warden of All Souls College, Oxford.

Henderson attended Stowe School and went on to become president of the union while at Hertford College, Oxford. The combination of Oxford and Bloomsbury created in Henderson and others an exhibitionist style which he described as a "common tendency to showiness, eagerness to amuse, eclectic taste with particular response to architecture, clothes-consciousness, the horror of the dim." These were qualities evident in him throughout his life. He missed the Second World War because of tuberculosis, instead joining the Foreign Office. There he came into contact with the Foreign Secretary Anthony Eden, the first of many senior figures he was to work with: they included Rab Butler, Ernest Bevin, Harold Wilson, Alec Douglas-Home and James Callaghan.

In the years that followed he attended important conferences in Potsdam and elsewhere, working in Whitehall and specialising in relations with the Soviet Union between postings to Vienna, Madrid, Santiago, Athens and Poland. During his spell in Greece he met his future wife, Mary Barber (née Cawadias), who had been a Red Cross nurse and was arrested by the SS and condemned to death for assisting the Allies. She later became a war correspondent for Time and Life magazines.

She remembered the young Henderson she met as: "Very tall and thin, with an easy, natural, untidy chic; a crumpled US seersucker suit and a copy of Thucydides' History of the Peloponnesian War stuffed in his pocket".

They married in 1951. It was said of her: "She was much more than an accomplished hostess: she was knowledgeable, shrewd and determined, with a wide experience of people and the vanities and foibles of human nature. He would have been foolish to discount her opinion on the various ministers and luminaries in the countries in which she lived."

As ambassador to Bonn in the early 1970s, Henderson became a committed European, convinced that the UK could not stand alone and should be part of the developing system. In this he was at one with the views of his close friend, Roy Jenkins. It was in Bonn that he first encountered Thatcher, on a visit as leader of the opposition. He was impressed by her memory and her concentration, saying of her: "She takes in everything one says, which means that one has to be very precise in the advice and information one gives her."

But later in life he was slightly less complimentary, saying that the Thatcher he met in Bonn was a different figure. "She changed terrifically," he said in an interview. "She was a very different person then and very different in her attitude to the direction of foreign policy than she became later. As far as I can guess she wasn't remotely anti-European in those days. In fact she liked my despatch theme, which was that we ought to take more part in Europe. All prime ministers want to play in the world's game because that's where they can get a lot of limelight."

His next posting, to Paris in 1975, confirmed his status in the upper echelons of British diplomacy. He bridled somewhat at what he saw as Gallic arrogance and the Parisian view that French culture and political influence were superior to those of Britain. He and Mary succeeded, however, in a counter-offensive, at least on the gastronomic level, by providing sumptuous banquets whose flair and elegance were the talk of all Paris. They succeeded in repulsing Foreign Office efforts to have their entertainment budget cut back.

Henderson's reference to Thatcher liking his despatch may have led to the extension of his career and his involvement in the diplomatic front line of the Falklands War. As he prepared to leave Paris and go into retirement he indulged himself with a farewell tour d'horizon memo, as ambassadors often do. This went much further than most, however, dwelling on the theme of British decline and describing his country as "poor and unproud".

Henderson later wrote: "There was no concealing our weakness by the mid-'60s, and a high moral tone proved to be all the more difficult to sustain in the face of dwindling material means."

The document was circulated throughout Whitehall and almost inevitably leaked to the press, where it was widely quoted in support of the Thatcherite theme that under Wilson and Callaghan Britain had lost much of its pride. The surprise came when Thatcher revealed that she thought of Henderson as more than simply a useful source of electioneering material.

She might have dismissed him as simply another Foreign Office europhile of the type she detested, fairly obviously a "wet" and extravagantly dining for Britain in decadent Paris. Instead, she had him elevated to the post of ambassador to Washington, one of the glittering prizes of the British diplomatic service. He and Mary, surprised and delighted, were a hit – both in political and culinary terms – especially after Ronald Reagan became president.

Thatcher and Reagan were developing their own relationship but Henderson, too, effectively cultivated the new president, saying of him: "He is a difficult man to oppose because you can-not hold anything against him personally. He is without cynicism or malice."

When Argentina invaded the Falklands, Britain pressed Washington for help in terms of intelligence, logistics and other resources, since apart from anything else the sheer distance to the islands was so huge.

Henderson was, at first sight, not a natural wartime representative, but he threw himself into the role and was generally reckoned to have been effective in mobilising US opinion to the British side. The irony was that much of his career had been concerned with post-war Europe, and with building political and economic structures designed to ensure that continental conflict would not break out again.

During the Falklands crisis he was required to put aside his sense of British decline and to project the idea that Britain had both the means and the willpower to re-take the islands from the Galtieri regime. At the same time he was required to extract as much help as possible from the US, which in itself was an implicit recognition of the fact that Britain might well find it impossible to win the war alone.

The lifetime diplomat might have seemed ill-suited to the task of defending a military campaign, yet he allowed no sign of any personal doubts to emerge.

In later life he wrote a series of books, often delightfully indiscreet, on the interplay of politics, diplomacy and personalities, which included deft pen-portraits of major figures. He was irritated by Whitehall attempts to halt publication of one of these. He recalled: "When I submitted my text to the Foreign Office for approval it was immediately banned – the whole thing was completely banned.

"I got a very severe letter saying that if I went ahead I would be carrying out one of the most deleterious acts of a public servant imaginable. The only example of a transgression I ever had was that at the time of the Falklands War I described Mrs Thatcher as 'looking tired'. It seemed to me to be so absurd."

David McKittrick

John Nicholas Henderson, diplomat, author and company director: born 1 April 1919; Assistant Private Secretary to the Foreign Secretary, 1944-47; HM Embassy, Washington, 1947-49; Athens, 1949-50; Permanent Under Secretary's Department, Foreign Office, 1950-53; HM Embassy, Vienna, 1953-56; Santiago, 1956-59; Northern Department, Foreign Office, 1959-62; Permanent Under Secretary's Department, 1962-63; Head of Northern Department, Foreign Office, 1963; Private Secretary to the Secretary of State for Foreign Affairs, 1963-65; Minister in Madrid, 1965-69; Ambassador to Poland, 1969-72, to Federal Republic of Germany, 1972-75, to France, 1975-79, to Washington, 1979-82; Keeper of the Privy Seal of the Duke of Cornwall, and Member of Prince's Council, 1985-90; Chairman, Channel Tunnel Group, 1985-86; KCVO 1991; married 1951 Mary Barber (died 2004, one daughter); died London 16 March 2009.